To quote
"Death Inc is a new RTS and Business Sim set in old England. Play as a freelance Reaper, and unleash the plague on the 17th century. Ransack farms, ports and cities; topple knights and noblemen, as you battle to increase your horde.

Back in the Nether, use your harvested souls as currency and expand Death Inc's base of operations. Hire underlings, build new rooms and refresh the decor."

Unlock new units, upgrades and special abilities.

The game allows you to do research in your basement business lair and unlock special abilities that make your battles even crazier and spectacular:

some examples from the site.

-Need to disrupt enemy? Just send some plague rats their way.
-Farmers refusing to play along? Sabotage them with exploding livestock.
-Villagers cowering indoors? Get them outside with a Bubonic lunch.
-Long range attack required? Unleash the pigeon pox! Those filthy sky vermin will be dive-bombing in no time.